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Activation of the Egg

The activation of the egg is brought about after the entry of the sperm. A series of changes take place in the egg after the acrosomal tubules contact the egg plasma membrane.

(1) Fertilization Cone. When the acrosome filament comes into contract with the egg membrane the cytoplasm moves up the acrosomal filament membrane. This result in the formation of an elevation called the fertilization cone.

(2) Fertilization Membrane. In the sea urchin the vitelline membrane surrounding the egg is 150-200A thick. It consists of two closely applied unit membrane, each 75A thick. The inner membrane appears to grow around the cortical granules. When the sperm comes into contact with the vitelline membrane the cortical granules explode, and their mucoprotein material becomes applied to the outer membrane. The membrane increases in thickness to 800A and is now called the fertilization membrane. It separates from the inner membrane by accumulation of fluid between the two membranes. Its function is to prevent the entry of sperms.

(3) Increase in Oxygen Consumption. In general, an increase in oxygen consumption can be demonstrated in eggs if precautions are taken to maintain a favorable environment of the gametes.

change in oxygen

(4) Breakdown of Polysaccharides. It unfertilized sea-urchin eggs polysaccharides are built up. Immediately after fertilization a rapid breakdown of polysaccharides takes place. There is a corresponding increase in lactic acid, showing that metabolic activity has increased.

(5) Increase in Protein Synthesis. It has been found experimentally that the rate of amino acid incorporation in fertilized eggs is more than twice that of unfertilized eggs. This indicates that protein synthesis increases after fertilization. Unfertilized eggs contain active ribosome. Messenger RNA is also synthesized before fertilization, probably in the oocyte. Although both active ribosomes and messenger RNA are present in the unfertilized egg, the rate of proteins synthesis is low.

(6) Hexose Phosphates which are very low in the unfertilized egg increase considerably after fertilization.

(7) Dehydrogenase Activity also increases. This indicates that enzymes are released, probably due to deinhibition.
The activation of the egg is considered to be a condition of deinhibition. At a certain stage of development the female gamete becomes or inhibited. If it is not fertilized it undergoes cytolysis, and dissolution.

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